Plastic Rack vs The RF Monster

[Harry Bissell]

Yo Jeremy:
    If you are talking about "analog" oscillators they are nowhere near radio
frequency. A properly designed circuit will not be affected. A high gain/low
noise circuit like a mic preamp (1-10mV) should be in a metal case. Most VCOs
etc have output levels like 5-10 volts. It would take a hell of a radio signal
to stomp you.
    A concern might be 60Hz AC "Magnetic" fields. The distinction here is how
the field attacks the circuit. High frequencies get in by capacitive coupling
(or by beating with an oscillator of like frequency, or picled up by a diode
or similar part in a high gain circuit). Capacitive coupling can be stopped
dead in its tracks by shielding (and heavy-duty aluminum foil glued to the
inside of the case would do just fine). Magnetic coupling works better at low
frequencies, and can penetrate aluminum with ease. A steel case (or even
better, a nickel alloy called Mu-metal for its very high permeability, or
ability to contain a magnetic field) will be needed to stop magnetic pickup.
Audio transformers and coils with iron cores are VERY OFTEN shielded with
Mu-metal. It kills hum pickup. Again. this will likely be a problem only in
very high gain circuits.
    Your friend has a point... Metal is superior in shielding to plastic. But
my guess is that it would be better to use the plastic for all its advantages
(light weight, impact resistance, never needs paint, and is very cost
effective (we're talking about $60 for a four to six space rack). Go for
plastic, I bet you can't tell (or measure) the difference.
    P.S. If you live right "under the guns" of a major league radio
transmitter, go for metal. I lived in a house SO BAD that even metal wouldn't
stop it. Eventually I threw out the inferior circuits and got some that
worked.
    
The NATIONAL SEMICONDUCTOR tips for "audio rectification, or how come my Phono
detects AM (from the 1977 Audio data book)
* Reduce input impedance  (makes it harder for the noise to drive)
* Place capacitor to ground close to input pin or base (~10-300pF)
  (gives the noise an easier path to ground - instead of your input)
* Use Ceramic capacitors  (they have excellent high freq performance)
* Put ferrite bead on input lead close to device input 
  ( soaks up high frequency waves like a sponge)
* Use RF Choke in series with input (~10uH)
  (Ahhh... now we're desperate because the choke might pick up 60Hz hum)
* Pray (recommended for everybody, not just techs)

(note: comments in parenthesis are mine, not National's)

Remember the famous line from the film "The Graduate"
"I have one word for you....... Plastics"     :-) Harry


Jeremy <jmartin at ixks.com> wrote:
>Jeremy: www.skbcases.com  look for "roto-rack" you can buy them from Guitar
>Center. They are ideal. lightweight, plastic. I'm re-doing my modular in
one. 
>:-) Harry

A friend of mine adviced to get a metal case, as it gets rid of noise
interference from radio frequencies that are similar to the oscilators
frequencies. I take it it is not that big of a deal if you recommend a
plastic box? 

Thanks!

___________________________________________________________________________

      Jeremy Martin - jmartin at ixks.com ; IXKS, Inc. - Internet Kansas
                    785.842.9996 - http://www.ixks.com
                  P. O. Box 1761, Lawrence Kansas, 66044
____________________________________________________________________________


____________________________________________________________________
Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.